Cushioning devices are used to protect railcars and lading from impacts during coupling and train action events. Cushioning devices are commonly mounted between the frame of the railcar and couplers on the ends of the cars to reduce the high forces caused by impacts.
Impacts applied to railcars result in high forces applied to the coupler in both the buff direction and the draft direction. "Buff" is a term in the rail industry used to describe the movement experienced by the coupler when it is moved towards its associated railcar. "Draft" is a term in the rail industry used to describe the movement experienced by the coupler when it is moved away from its associated railcar. A buff impact moves the coupler towards its associated railcar. A draft impact moves the coupler away from its associated railcar.
Railcar impacts are conventionally cushioned by hydraulic cylinders. In one type of hydraulic cylinder, pressurized gas in hydraulic fluid in the cylinder biases the piston to a fully extended position. If a draft impact occurs while the piston is fully extended, the device is unable to cushion the impact because the piston cannot move further in a draft direction. In another type of gas charged hydraulic cushioning device, an externally mounted spring prevents pressurized hydraulic fluid from fully extending the piston and holds the piston in a neutral position. The piston can move from the neutral position in response to either buff or draft impacts. The external spring increases the size of the cushioning device and makes installation difficult. The external spring is exposed to dirt and other environmental contaminants that can adversely affect operation of the cushioning device. The restoring force generated by the external spring acts along a line of force spaced away from the line of action of the cylinder itself, and may cause uneven or accelerated wear of moving components.
My U.S. patent application Ser. No. 09/009,098 filed Jan. 20, 1998 and assigned to the assignee of the present application discloses a hydraulic cushioning device having an internal ring spring confined between the piston and the front head of the cylinder. The ring spring holds the piston in a neutral position to permit hydraulic cushioning of buff impacts and combined hydraulic and mechanical spring cushioning of draft impacts. A collapsible ring keeper holds the ring spring together and collapses with the ring spring. The cushioning unit described in my application has advantages over prior hydraulic cushioning units which hold the piston in a neutral position a distance from the front head of the unit. The ring spring is substantially linear along the draft collapse stroke. Also, the ring spring is relatively long and increases the length of the cushioning unit.
For some cushioning applications it is desirable to provide a spring which holds the piston of the cushioning unit in a neutral position and which also provides an increase spring rate at the end of the draft stroke to absorb large draft impacts. Additionally, it can be desirable to provide a more compact cushioning unit which holds the piston in a neutral position in order to reduce the size of the draft sill pocket and manufacture the cushioning unit more economically.
Thus, there is a need for an improved gas charged hydraulic railcar cushioning device of the type which includes an internal spring holding the piston in a neutral position where the spring provides improved cushioning in response to high draft impacts and facilitates reducing the size and cost of the cushioning unit.